Production of subsurface oil and gas deposits is typically from deep wells drilled through high pressure formations. Lining the wellbore from top to bottom is steel casing, anchored by a sheath of cement that is securely and circumferentially bonded to both the casing and the wall of the wellbore. The control of the underground pressure is an important factor in both the drilling and the production operations. Improper well control can result in the sudden and uncontrollable release of hydrocarbons. Such an incident is commonly referred to as a blowout, resulting in dangerous and costly delays in the drilling or production operations. Such blowouts often catch fire or are otherwise hazardous at the surface, necessitating the drilling of a relief well from a remote surface location to effect a subsurface kill.
Generally, the relief well must approach the blowout well within some reliably known close distance within a limited depth range. This kill distance varies from a maximum of about 25 feet in a high permeability limestone formation down to a direct intersection where the kill is made casing-to-casing or in a tight sandstone.
Conventional directional drilling practice determines the direction and position of boreholes by running hole direction surveys and processing the directional data from hole position. The directional measurements are commonly made using a magnetic compass and inclinometer. The standard deviation of these directional measurements is not accurately known, but is probably greater than the resolution of the instruments. There are other random errors due to instrument friction and mounting instability, tool alignment in the borehole, gravity pertubations, magnetic variation uncertainty, magnetic disturbance of the drill pipe, etc. In addition to these random errors, there are bias errors. The largest bias error is probably in the compass reading caused mostly by poor calibration and the drillpipe magnetic disturbance. The total directional bias error can range up to a few degrees or even more.
Presently, relief well drilling involves directionally drilling the relief well down to the blow out well position calculated from the directional survey. However, when a direct intersection between the relief well and blow out well is required, such a "hit or miss" operation of relief well drilling can become quite time consuming and costly where several relief wells are drilled before a direct intersection is achieved.